Contains screenshots of places where I found most of the collectables to help you get your achievement.
Collectibles on Hellmark Station
There's are two canteens you can walk behind. Behind one of them you'll see a skull. You can shoot it either from the left, or walk around the counter and shoot it from there. Ofc glass is bulletproof.
A skull can be found on top of a ladder by Jittery Joes, close to main, open hall you'll find yourself on often.
On said hall there's a skull on top of uh... crucified? corpse. It's opposite that big, metal support structure.
Impaled onto fence on gate to platforms 3 and 4.
In this small glass enclosure around a lava pit there's opening on one side. Skull is underneath the bench.
On drawers in staff room with knocked down wall.
On a rack in what I believe is one of storage corridors?
Upstairs, by a portal, on one of ATM machines.
In a room with exhibits and Kraken statue, a skull sits peacefully on a moose head.
In security room, hiding away under the desk.
Pub with snooker tables? Hail! Skull stands by beverages behind the bar.
In remains of a clothes shop, on a shelf towards the middle of the shop.
(Pardon the gorefiend limb in my face) a skull can be found in one of the kitchens, tucked away in the metal sink.
You know that funny hell stone that goes brrr and takes away 20hp? There is a skull on top of the one growing out of the catwalk going above 3 platforms.
Next to underground ticket gates, behind an ATM.
There's more than one bathroom you can access, this is the one with knocked down doors and wall. Skull sits on sinks.
I found at least 2 more that I forgot to take a screenshot of, I found them in rooms neighbouring skull on moose, but the ones listed above should be enough to get your achievement. You need 10, right? I think so.